1. Field of the Invention
The present invention relates to a method and apparatus for controlling an engine using a cam signal.
2. Description of the Related Art
Conventional engine control units for vehicles have carried out engine control, such as fuel injection control and/or ignition timing control, in synchronization with rotation of an engine crankshaft.
Specifically, a crank signal is output from a crankshaft sensor. The crank signal consists of a train of crank pulses whose same-directed edges correspond to angular positions of the crankshaft as it rotates. A pulse cycles of the pulse train corresponds to a predetermined angular interval of the crankshaft rotation. When the detected crank signal is input to the engine control unit, the engine control unit multiplies a frequency of the crank signal to generate a frequency-multiplication clock signal. For details, the frequency-multiplication clock signal consists of a train of clock pulses whose clock cycle is a positive integral submultiple of the pulse cycle of the crank signal.
The engine control unit increments an angular counter indicative of a rotational position (crank position) of the crankshaft in response to the multiplication clock signal in accordance with a crank-signal synchronized control program that has been stored in the engine control unit. The crank position of the crankshaft can be referred to as crank angle. This allows the engine control unit to grasp the crank position of the crankshaft with a resolution higher than that obtained based on the angular intervals of the crankshaft. The resolution obtained by using the angular counter can be represented as the value obtained by dividing an angular interval of the crankshaft by the number of the multiplication. When the count value of the angular counter coincides with a predetermined setting corresponding to a given crank position, the engine control unit carries out fuel injection or ignition. These operations of the engine control unit are for example disclosed in Japanese Unexamined Patent Publication No. 2001-200747 and U.S. Pat. No. 6,445,998B1 corresponding to Japanese Unexamined Patent Publication No. 2001-214790.
An engine control unit of this type includes a pulse-edge interval counter and a multiplication counter as means for generating the multiplication clock signal. The pulse-edge interval counter is configured to measure time intervals between active edges, such as trailing edges, of a pulse signal input thereto. The measured intervals will be referred to as pulse-edge intervals hereinafter.
The multiplication counter is configured to generate a frequency-multiplication clock signal in synchronization with the pulse signal; this frequency-multiplication clock signal consists of a train of clock pulses whose clock cycles correspond to time intervals obtained by dividing the measured pulse-edge intervals by the predetermined number of multiplication.
Specifically, when the crank signal is input as the pulse signal to the pulse-edge interval counter, the pulse-edge interval counter measures pulse-edge intervals between active edges of temporally adjacent crank pulses in the input crank signal. The multiplication counter generates the frequency-multiplication clock signal consisting of a train of clock pulses whose clock cycles correspond to time intervals obtained by dividing the measured pulse-edge intervals by the predetermined number of multiplication.
Note that, in the Japanese Unexamined Patent Publication No. 2001-200747 and U.S. Pat. No. 6,445,998B1, the direction of each of the same-directed pulse edges of the crank pulses in the crank signal is the same as that of each of the active edges of a pulse signal input to the pulse-edge interval counter. This allows the crank signal to be directly input to the pulse-edge interval counter. If the direction of each of the same-directed pulse edges of the crank pulses in the crank signal is different from that of each of the active edges of a pulse signal input to the pulse-edge interval counter, the crank signal can be input to the pulse-edge interval counter with its level being inverted.
In engine control units using the crank signal for engine control, if the crank signal fails to be input to the pulse-edge interval counter due to, for example, a break in wires connecting the crankshaft sensor and the pulse-edge interval counter, the frequency-multiplication clock signal cannot be generated. This may cause the count operation of the angular counter to stop so that the engine control unit may not carry out engine control in synchronization with rotation of the crankshaft.
Engine control units using the crank signal for engine control have therefore stored therein a fail-safe control program in addition to the crank-signal synchronized control program. The fail-safe control program is designed to cause a computer, such as a microcomputer of the engine control unit, to carry out engine control, such as fuel injection control and/or ignition timing control, based on a cam signal output from a camshaft sensor. The cam signal is configured to vary between its high and low levels depending on rotation of an engine camshaft.
Specifically, when detecting an abnormality of the crank signal, the engine control unit runs the fail-safe control program in place of the crank-signal synchronized control program to allow the vehicle to be driven in a limp-home mode, namely, driven home slowly.
On the other hand, U.S. Pat. No. 5,671,145 corresponding to Japanese Unexamined Patent Publication No. H07-310582 discloses a method of generating a simulated crank signal based on a cam signal detected by a cam sensor in an event of a failure of a crankshaft sensor, and of controlling the engine based on the simulated crank signal.
Install of the fail-safe control program in an engine control unit in addition to the crank-signal synchronized control program may reduce free space therein and increase the number of person-hours required to develop engine-control software including the control programs.
The publication U.S. Pat. No. 5,671,145 discloses generation of the simulated crank signal based on the cam signal, but may not disclose specific configurations and operations required to generate the simulated crank signal. Moreover, in order to concretize the techniques disclosed in the Publication U.S. Pat. No. 5,671,145, a dedicated circuit is probably required to generate the simulated crank signal based on the cam signal, which may increase hardware configuration in the engine control unit.